The nitrogen autoclave process has been used to produce polymer foams commercially for many years and is described in some detail in patents U.S. Pat. No. 3,640,915 and GB 899,389 as well as in books such as Polymeric Foams, edited by S. T. Lee and D. Scholz (CRC Press, 2009) and Handbook of Polymer Foams, edited by D. Eaves (Rapra Technology Limited, 2004). The process typically comprises three stages; in the first stage a solid polymer article is produced, in the second stage gas is absorbed into the polymer under high pressure and at elevated temperature and in the final stage the article is expanded to produce a foam.
In the current state of the art, extrusion has generally been used to produce the solid polymer article of the first stage as this is a cost effective method of producing a flat sheet that is relatively easy to handle and allows efficient loadings in the autoclaves required for the subsequent stages of the foaming process. However, whilst the extrusion process does produce a solid polymer article that can be easily and efficiently loaded into the autoclaves, it does limit the form of the final product to an essentially two dimensional rectangular sheet of uniform thickness, and whilst the sheet form of the product may be suitable for many applications it has become increasingly clear that for many high volume applications, such as the midsoles used in running shoes, a near-net-shape article of varying thickness would be highly desirable.
Various processes which employ pre-forms are disclosed in US2003/0046832; U.S. Pat. Nos. 6,776,939; 6,787,079; 7,052,634; 7,464,428; 8,906,280; GB2014153A and US2014/0259801. Other processes are disclosed in U.S. Pat. Nos. 6,993,858; 7,146,751 and 6,439,536.
Other processes for manufacturing three dimensional polymer foam articles are also known to those skilled in the art, however, these do not typically produce articles of uniform density or isotropic cell structure. For example, bead foam processes typically result in density variations where there is a change in thickness of the foamed part, and thermoforming processes typically result in an anisotropic cell structure being created, even if the initial cell structure of the foam was isotropic.